Monocrystalline tubular semiconductor



MONOCRYSTALLINE TUBULAR SEMICONDUCTOR Filed Dec. 50, 1954 FIG. 2

INVENTOR. KU RT LEHOVEC BY cmm wg HIS ATTO NEYS MONDCRYSTALLINE TUBULARSEMICONDUCTOR Kurt Lehovec,Williamstown, Mass., assignor to SpragueElectric Company, North Adams, Mass, a corporation of MassachusettsApplication December 30, 1954, Serial No. 478,685

4 Claims. (Cl. 148-33) This invention relates to new and improved monoa:ed Stes Patent 9 crystalline semiconductive structures and moreparticularly to novel embodiments of monocrystalline germanium andsilicon having electrical and chemical applications.

Some chemical reactions require containers that are quite inert atelevated temperatures and are able to withstand high pressures. Althoughthe prior art bombs used for this purpose are readily constructed towithstand pressures, they are generally made of relatively active metaland are accordingly subject toattack by the reactants, particularly atelevated temperatures, so that the reactants frequently becomecontaminated.

Among the objects of this invention is the provision of improvedcontainers in which the above difiiculty is minimized. It is a furtherobject of this invention to provide monocrystalline tubularsemiconductors that can be used to provide either containers orelectrical translating elements for rectifiers, transistors or the like.

The above as well as additional objects and advantages of the presentinvention will be more apparent from the following description ofseveral of its exemplifications taken in conjunction with theaccompanying drawings, wherein:

Figure 1 is a perspective view of a container illustrating the presentinvention;

Figure 2 is a sectional view of an open-ended tube representing anotherembodiment of this invention; and

Figure 3 is a perspective view of a translator element typical of thepresent invention.

According to the present invention a tube of semiconductor material suchas silicon or germanium is provided in the form of a single crystal.These semiconductor materials are relatively inert in the chemicalsense, more so than the metals generally used for reactor bombs. Inaddition, the fact that they are in the form of a single crystal makesthem exceptionally strong and easily capable of resisting the highpressure that may be applied to chemical reactions. Either of thesematerials in a wall thickness of for example, will withstand quite a fewatmospheres of pressure.

The tubular construction can be provided by either drilling out thecenter of a suitably dimensioned rod, or by directly growing thesemiconductor crystal around an elongated core. Referring to Fig. 1,there is shown a container in the form of a tube having one end open andone end closed. When used under pressure, the open end can be covered asby a semiconductor slab or lid, and the contact points of the lid to thebomb can be fused together by local heating. Alternatively, the openmouth of the container can be provided with a securing element, such asexternal threads or lugs against which a correspondingly-shaped portionof the lid can be secured so as to form a cap. Any soft material inertto the reactants can be used as a gasket between the container and cap.For temperatures of about 300 C. or below, polytetrafluoroethylene orlead sheet make suitable gaskets.

The growing of a container in the form shown in Fig. 1 can be effectedin the manner described in U. S. Letters Patent 2,631,356, granted March17, 1953, except that a solid inert core is inserted through the seedcrystal and pulled outwith it as the crystallization growth progresses.As pointed out in that patent, this is a conventionalway of growingsemiconductor crystals, and the external diameter of the growing mass isreadily controllable. By selecting a core of a material that has athermal expansion coefficient greater than that of the semiconductormaterial, .the withdrawal of the core from the grown mass ofsemiconductor is simplified. A carbon rod makes suitable core for thegrowth of a cylindrical germanium crystal. Best results are obtained,however, if the grown mass containing the core is subjected to arelatively low temperature, -70 C. for example, to facilitate thewithdrawal. Such a temperature is conveniently provided by Dry Ice. Thestrength of the container can also be improved by smoothing its externalsurfaces, as by a machining and polishing operation.

Instead of having the crystal grown in such a way as to close one end ofthe tube, the pulling of the mass can be controlled so that the growthis discontinued before the end of the core is reached. In other words,the growing mass can then be quickly removed from the liquid material toprovide a tube which is open at both ends. When such a tube is used as areactor bomb, lids or caps can be provided on both open ends.

A feature of the present invention is the fact that the tubularconstruction is also readily adapted for providing electricaltranslating elements such as are used in rectifiers or transistors. Tothis end, the tube can be provided with an electrical conductivityjunction. When the tube is grown from a liquid in the manner indicatedabove, the junction is readily furnished by merely doping the surface ofthe core with the appropriate type of impurity. At the high temperatureof the growing operation, this impurity tends to diffuse into thesemiconductor material from the core.

After the semiconductor tube has been grown, impurities providing thedesired type of conductivity upon incorporation into the semiconductorcan be introduced as follows: The impurity is introduced into the spaceeither inside or outside of the tube, and is diffused into thesemiconductor by heat treatment. It is recommended to introduce theimpurities in gaseous form; e.g. in an n-type germanium tube, boronhydride is passed at elevated temperatures, thereby rendering the innerside of the germanium p-type by diffusion of boron into the germanium.

Fig. 2 shows a single crystal semiconductor tube having an electricalconductivity junction provided in the above manner. The body of materialhas its external portion 10 of one type of electrical conductivity suchas the N-type, provided by an extremely small. content of antimony, forexample. The balance or inner portion 12 of the body can have a P-typeelectrical conductivity, as for example by reason of the diffusion ofindium in small concentrations. At the limit of diffusion, there is ajunction 14 where the different electrical conductivities meet.

The tube of Fig. 2 can be sliced transversely into thin rings which makesuitable bodies to which contacts can be connected for making arectifier or transistor. Lowresistance, or so-called ohmic contacts canbe applied, as by soldering, to the respective portions 10, 12 of thetube, before or after it is sliced. Point contact electrodes can also beconnected to the semiconductor slices whether or not they have ajunction.

More than one junction can be provided in the tube or the individualslices, as by diffusing an additional impurity into the body from itsexternal or internal face. Where the second diffusion takes place fromthe same face as the first dilfusion, the second should not penetrate asfar as the first. Fig'. 3{shows'a slice of the above typeinwhich twojunctions are present. Here the successivezones of the semiconductor areidentified as 20, 22 and'24'Withthe intervening junctions 26 and 28. Thezo'n es can have either the N-P-Nor P- NP sequence, with suitableconnections being provided as indicated above to completea'corresponding type of transistor.

The presence or absence of a junction in a semiconductqr body has noeffect on its ability to satisfactorily res-ist-pressuresas well aschemical attack when used as a container or bomb for chemical reactions.7 Byway of example, the drilling out of a rod, referred to above,'canbeefiected either by a mechanical drilling arrangement using a standarddrilling tool, or it can be ac- Tc omplished electromechamically. Theelectromechanical dissolution of germanium, for example, is described inthe December 195 3- issue of the Proceedings of the I.R.E., volume 41,pages 1706+1708, and all that is. necessary is that a germanium rod beconnected asan anode with respect to a cathode of pointed 'form with thepoint advancing into the germanium as it is dissolved. Inasmuch as theelectrolyit ic current will concentrate on the portions of the germaniumclose to the point of the cathode, the anodic dissolution will proceedin a penetrating fashion to create "an opening to 20 times the diameterof the pointed cathode, depending upon the distance that is maintainedbetween the cathode and the anode. To make a container for chemicalreactions, a'suitable internal diameter is /zf or even less. Fortranslating elements, internal diameters as little as- A "or even 3 arepreferred. Asmany-apparently widelydifierent embodiments of thisinvention maybe made without departing from the spil-itandscope hereof,it is to be understood thatthe invention is notlimited to the specificembodiments hereof. except as defined in the appended claims. What isclaimed is:

LA bomb type chemical reactioncontainer in the 4 form of a tubularsingle crystal of semiconductor material of the class-consisting ofsilicon and germanium, the tube having a closed bottom.

2. A process for producing a monocrystalline tubular semi-conductor of amaterial of the class consisting of silicon and germanium whichcomprises inserting a core of an inert material havinga-thermalexpansion co efiicient greater than the semi-conductor material into aseed crystal of the semi-conductor material, growing a crystal of thesemi-conductor material on the core, and s'ubjectingthe grown crystaland core to a low tempera ture to facilitate removal of the core.

3. A tube of semiconductor material of the class coni sisting of siliconand germanium, said tube being inthe form of an elongated singlecrystal, at least one end of said tube being closed to provide a bombtype chemical reaction container.

4. An annular semiconductor body of a material of the .class consistingof silicon and germanium, said body being in the form ofan elongatedannular single crystal, saidbody having atleast one concentricallydisposed ringshaped-electrical'conductivity junction extendingtherethrough, said body having at least one closed end to provide a bombtype chemical reaction container capable of dilfusing conductioinimpurities from saidring-shaped junction.

References'Citedin the file of this patent UNITED STATES PATENTS OTHERREFERENCES B1 1 Cr tal r w h, page 508, 1951-

2. A PROCESS FOR PRODUCING A MONOCRYSTALLINE TUBULAR SEMI-CONDUCTOR OF A MATERIAL OF THE CLASS CONSISTING OF SILICON AND GERMANIUM WHICH COMPRISES INSERTING A CORE OF AN INERT MATERIAL HAVING A THERMAL EXPANSION COEFFICIENT GREATER THAN THE SEMI-CONDUCTOR MATERIAL INTO A SEED CRYSTAL OF THE SEMI-CONDUCTOR MATERIAL, GROWING A CRYSTAL OF THE SEMI-CONDUCTOR MATERIAL ON THE CORE, AND SUBJECTING THE GROWN CRYSTAL AND CORE TO A LOW TEMPERATURE TO FACILITATE REMOVAL OF THE CORE. 